1. Field of the Invention
The present invention generally relates to welding guns suitable for use on automated assembly lines. More specifically, this invention relates to a welding gun having a pair of electrode tips and an equalizing linkage mechanism that equalizes the movement of the electrode tips towards a workpiece and the pressure applied by the electrode tips to opposite sides of the workpiece while the workpiece is being welded.
2. Description of the Prior Art
Electric welding guns, and particularly resistance welding guns, are widely used in mass production to join workpieces to form permanent assemblies. Resistance welding guns are especially suited for use on automated automobile assembly lines where body panels, such as doors and hoods, and their support structures are welded together to form subassemblies, which are then welded to the frame or unit construction of an automobile. Unit construction automobile bodies are themselves complex structures of stamped sheet metal sections which must be welded together to form a framework to which the outer body panels are attached.
Resistance welding is a process which, through the application of heat and pressure, coalesces two or more metal sheets or panels, without the use of fluxes or filler metals, to form a permanent joint. The necessary heat is generated by the resistance of the metal sheets to the flow of an electric current through the metal sheets. Conventional resistance welding guns include electrodes which serve as terminals for an electrical circuit. The electrodes typically are formed from copper which is alloyed with such metals as molybdenum and tungsten to be electrically and thermally conductive, while at the same time having sufficient strength to withstand the high application forces associated with holding the metal panels for welding. During the welding process, the electrodes are first brought into contact with opposite sides of the sheets to be welded. An electric current as high as about 20,000 amps is then passed through the electrodes and through the sheets. The magnitude of the electric current must be sufficient, but not excessive, to locally heat and melt the panels so as to fuse the panels to each other between the points where the electrodes make contact with the panels. The current may be direct or alternating, depending upon the particular requirements of the application and the electrode material used.
The resistance welding process conventionally includes the application of pressure to the workpiece with the electrodes. The use of force applied by the electrodes is often beneficial in that it assures a firm contact between the panels being welding, and also assures that proper electrical contact between the panels and the electrodes is made. However, excessive force must be avoided where the workpiece being welded is thin and therefore susceptible to deformation from excessive impact loads or asymmetric clamping loads. This is particularly true in the automotive industry where demands for lighter weight cars has resulted in the use of body panels which traditionally were about 0.035 inches thick, but recently have been as thin as 0.023 inches. The recommended electrode force for welding these panels can be as high as 700 pounds, making deformation a reoccurring problem on automobile assembly lines.
Where large welding guns are used, such as those controlled by robots on an assembly line, it has been conventional practice to construct the welding gun to be self-centering. Typically, this entails a pair of arms pivotably mounted to a base, wherein each of the arms has a first end which is fitted with an electrode. A cylinder is attached to the opposite ends of the arms to rotate the arms relative to each other. In that the arms are free to pivot together on the base, it is intended that one of the electrodes comes into contact with the workpiece prior to the second electrode as the cylinder is extended to bring the electrodes together. Once the first electrode has made contact, it forces the second arm to unilaterally complete the travel of the arms toward the workpiece as the cylinder continues to extend, such that a self-centering capability to the welding gun results. The initial impact is a combination of the force generated by the cylinder and the inertia of the welding gun as it is forced to rotate into position by the first electrode. While this initial impact has not typically caused excessive deformation in the workpiece, with body panels in automobiles getting thinner, even these loads have been found to unacceptably deform the body panels.
Several approaches for minimizing the adverse effect of the clamping force applied by the electrodes have been suggested in the prior art. An early approach taught by U.S. Pat. No. 1,980,228 to Rogers was to provide adjustment to the force applied by a pair of compression springs. Rogers disclosed an eccentric pin which, when rotated, adjusted the gap between a pair of electrodes when the electrodes were subject to the force of the springs urging the electrodes together. However, the welding gun taught by Rogers is not well suited for use on an automated assembly line in that the actual force imposed by the electrodes on the workpiece depends upon possible variations in thicknesses of the workpiece.
A more recent approach is taught in U.S. Pat. No. 4,771,160 to Pitsch et al., which teaches the use of an elastic member which resides between a pair of arms which have a pair of opposed electrodes. The resilient member serves to both impose a force at the electrodes upon the workpiece, while also serving to absorb some of the impact. However, nothing prevents one of the electrodes from impacting the workpiece prior to the other electrode which, as noted above, is a more recently recognized problem.
An approach taught by U.S. Pat. No. 5,036,175 to Umeda is specifically directed to equalizing the force imposed by the two electrodes. Umeda uses an intermediate linkage between a pair of arms to urge one of the arms away from the workpiece under the opposing force of a spring. As a cylinder begins to move the second arm into position, the linkage gradually releases the first arm, allowing the first arm to come into contact with the workpiece under the force imposed by the spring. Again, the movement of each electrode is not the same, allowing one electrode to impact the workpiece unopposed before the other electrode can be brought in to absorb the impact of the first. Finally, U.S. Pat. No. 5,099,099 to Saito teaches a welding gun in which only one arm is rotated while the other is held stationary. Saito uses a pair of linkages to define the movement of the moving arm, but teaches nothing that will ensure that the load will be equalized between the two arms.
From the above discussion, it can be readily appreciated that the prior art does not disclose a resistance welding gun which is adapted to provide both equalized movement and loading of the electrodes immediately preceding and during the time the electrodes come into contact with a workpiece. Nor does the prior art teach or suggest the advantages of impacting a workpiece simultaneously with both electrodes to prevent deformation of the workpiece by a combination of the force generated by the cylinder and the inertia of the welding gun as it is forced to rotate into position by the first electrode to contact the workpiece.
Accordingly, what is needed is an economical method for providing equalized movement and loading of a pair of resistance welding electrodes while the electrodes are brought into contact with the workpiece such that workpiece deformation is not caused by one electrode impacting the workpiece prior to the second, while also ensuring that the loads imposed by the electrodes on the workpiece will be substantially equal throughout the operation of the welding gun.